1. Field of the Invention
The present invention relates to methods for detecting a partial discharge in a high-voltage transmission and distribution system, and a partial discharge detection apparatus used therefor. More particularly, the present invention relates to a method for detecting a partial discharge in an insulator of a high-voltage power cable, for example, and to a partial discharge detection system used therefor.
2. Description of the Related Art
For example, if there is foreign matter or a void in a portion of the insulator of a high-voltage power cable, a partial discharge occurs at that portion. A discharge phenomenon occurs in which an electrical tree grows in the insulator until an electrical breakdown, such as a short circuit, occurs. The inner portion of an electrical tree is hollow and the amount of the partial discharge (measured in pC) increases as the electrical tree grows. Thus, it is possible to predict an electrical breakdown by detecting a partial discharge and monitoring the amount of the partial discharge.
Accordingly, monitoring a partial discharge is performed by applying a voltage to a high-voltage transmission and distribution system, such as a power cable. However, since a signal associated with a partial discharge is very weak, the partial discharge often cannot be detected due to noise contamination. Such noise may be caused by electromagnetic waves, engine sparks from vehicles, sparks from motors, or aerial discharges from bare electrical wires or wire terminals. In order to prevent the influence of such noise, a partial discharge is detected in a shield room, as shown in FIG. 5.
As shown in FIG. 5, the shield room is a room that is grounded, in which a high voltage is applied to a power cable 1 via a blocking coil 2. In addition, a detector 3 is attached to the power cable 1 to measure an electric current flowing through a circuit including a high-voltage capacitor 4, and to thereby detect the partial discharge. Here, to detect the partial discharge, an output signal from the detector 3 is transmitted through a resonant circuit 5 to remove commercial frequency components from the output signal, as shown in FIG. 6, thereby acquiring only a partial discharge signal associated with the partial discharge. The partial discharge signal is amplified by an amplifier 6 and then demodulated by a demodulator 7. Thus, a signal is obtained which has an amplitude corresponding to the amount of the partial discharge.
As described above, external noise caused by, for example, electromagnetic waves or engine sparks from a vehicle are removed by measuring the partial discharge inside the shield room. However, it is not possible to prevent noise caused by a motor attached to a step-up transformer inside the shield room or noise caused by aerial discharges from an terminal of the power cable. To remove such noise, for example, a detector 7 may be attached to the high-voltage capacitor 4 to differentiate the outputs from the two detectors 3 and 7, as shown in FIG. 7. In this case, the noise captured by the power cable causes electric currents to flow through the two detectors 3 and 7 in the same direction. Accordingly, the noise is cancelled out by differentiating the outputs from the two detectors 3 and 7, as shown in FIG. 8. On the other hand, when a partial discharge occurs, electric currents will flow through the two detectors 3 and 7 in opposite directions, as shown in FIG. 9. Accordingly, a high output corresponding to the partial discharge can be obtained by differentiating the outputs from the two detectors 3 and 7, as shown in FIG. 10.
In addition, as shown in FIG. 11, a technique for monitoring a phase angle region in which a partial discharge is likely to occur has been disclosed. This technique uses a property that a partial discharge and an aerial discharge are likely to occur in different phase angle regions of a signal at an applied frequency (commercial frequency) (e.g., see Japanese Patent Laid-Open Publication No. Hei 6-331686).
When monitoring a partial discharge in an in-service power cable, since the power cable may capture noise caused by, for example, electromagnetic waves, such noise must be removed. Another technique for detecting a signal associated with a partial discharge by monitoring a partial discharge in a frequency region at a low noise level has been disclosed, as shown in FIG. 13 (e.g., see Japanese Patent Laid-Open Publication No. Hei 6-308192). This technique utilizes the fact that a signal associated with a partial discharge has a flat frequency characteristic, as shown in FIG. 12.
Furthermore, a method for detecting noise, such as noise caused by engine sparks from a vehicle, using a detector 9 attached to an antenna 8 which is installed near the power cable is available, as shown in FIG. 14. This method utilizes the fact that noise is received by both the power cable and the antenna whereas a signal associated with a partial discharge is propagated only through the power cable. Thus, the noise can be cancelled out and a signal associated only with the partial discharge can be detected by differentiating the outputs from the detector 3 attached to the power cable and the detector 9 attached to the antenna 8, as shown in FIG. 15.
However, in the case of monitoring a partial discharge in the shield room, only a power cable that is not in service can be checked, and a power cable that is in service cannot be checked for a partial discharge. Thus, it is not possible to detect degradation of an in-service insulator.
Additionally, in the case of monitoring a partial discharge within the frequency region at the low noise level, it is difficult to select a detecting frequency because noise frequency characteristics may vary depending on the location of measurement. Furthermore, even when measurements are made at the same location, the noise frequency characteristics may vary over time. Thus, although a partial discharge can be detected during a particular period of time, it may be difficult to detect a partial discharge at all times. In addition, a frequency region at a low noise level is typically a high-frequency region. Thus, a long cable, such as a power cable, has greatly increased signal attenuation. Therefore, the sensitivity of the detection deteriorates when a partial discharge occurs in a distant place from the detection location. Furthermore, since the detection of the partial discharge is performed within a frequency region at a low noise level, the detecting frequency must be varied corresponding to a change of noise frequency characteristics, thereby increasing the complexity of the circuit configuration.
On the other hand, in the method for canceling out noise by installing the antenna, the detected pulse obtained from an object being measured and the detected pulse obtained through the antenna are often different both in level and timing. Thus, the method cannot easily distinguish noise from a partial discharge. Furthermore, the method has no effect on non-pulsed noise, such as electromagnetic noise. In addition, when the electromagnetic noise level is high or noise occurs very frequently, a signal associated with a partial discharge may be masked by the noise.